Problem: Define $f(x)=3x-8$. If $f^{-1}$ is the inverse of $f$, find the value(s) of $x$ for which $f(x)=f^{-1}(x)$.
Explanation: Substituting $f^{-1}(x)$ into our expression for $f$ we get  \[f(f^{-1}(x))=3f^{-1}(x)-8.\]Since $f(f^{-1}(x))=x$ for all $x$ in the domain of $f^{-1}$, we have \[x=3f^{-1}(x)-8.\]or  \[f^{-1}(x)=\frac{x+8}3.\]We want to solve the equation $f(x) = f^{-1}(x)$, so \[3x-8=\frac{x+8}3.\]or \[9x-24=x+8.\]Solving for $x$, we find $x = \boxed{4}$.